Cosmological simulations in pathological geometries and how to make them

by Balázs Pál (ELTE, HUN-REN Wigner RCP)

Friday 24 Apr 2026, 14:00 → 15:00 Europe/Budapest

Cosmological N-body simulations are fundamental tools for studying the non-linear evolution of large-scale structure, yet the vast majority adopt periodic cubic (T^3) boundary conditions. This choice breaks rotational invariance, prevents angular momentum conservation, and introduces artificial correlations at scales comparable to the box size. The StePS simulation framework and its companion initial condition generator stepsic overcome these limitations by compactifying open (R^3) and cylindrical (S^1 x R^2) domains via stereographic projection, providing an end-to-end pipeline for cosmological simulations that preserve the relevant continuous symmetries while maintaining radially varying mass resolution through a natural zoom-in configuration. The pipeline supports Lagrangian perturbation theory up to second order across spherical, cylindrical, and anisotropic slab geometries --- geometries that no other cosmological N-body pipeline supports. The first S^1 x R^2 LCDM simulation demonstrates that the cylindrical topology faithfully reproduces both linear and non-linear structure formation. This opens a pathway toward self-consistent studies of filamentary environments and anisotropic cosmological models in geometries that respect their intrinsic symmetries.